THE BRISTLE-WORMS. 



117 



differ from them in possessing a limy tube which can usually 

 be closed by an operculum, and in the presence of the so- 

 called " thoracic membrane," which is a delicate membrane 

 at either side of the thorax used in smoothing the inside of 

 the tube. There are a great number of Serpulids just as 

 there are of Sabellids. The conspicuous white limy tubes 

 are very common objects on shells and stones, both on the 

 rocks and among the wreckage flung on the beach, and are 

 familiar to most people, but the worms themselves are less 

 well known. In deep water Serpula itself is very common, 

 but on the shore 

 rocks a form called 

 Pomatoceros trique- 

 ter is the most fre- 

 quent. Notice the 

 distinct keel which 

 runs along the dorsal 

 surface of the tube, 

 and ends in a dis- 

 tinct spine over- 

 hanging its opening; 

 then select a few of 

 the largest speci- 

 mens you can find, 

 and place them, 

 with the shells or 

 stones to which they are attached, in a vessel with clean 

 water. After a period of patient waiting you will see a 

 crown of brilliant gills protruded, whose white ground colour 

 is relieved by splashes of crimson, orange, or blue. As the 

 filaments separate out in the water, notice that, as in 

 Sabellids, they arise in two clusters. Note further that in 

 one of the clusters the filament nearest the mid-dorsal 

 line is converted into a stopper, or operculum, borne 

 on a stalk. The corresponding filament at the other side is 

 aborted. If the worms be not alarmed, they will protrude 

 themselves far enough to show a collar like that of a Sabellid, 

 and the wavy thoracic membrane. In the thoracic region a 

 blue tint usually predominates. By carefully breaking the 

 tube it is possible to remove the worm without injury, so as 

 to display the whole body. Note then the general resem- 



Fio. 39 Serpula vermicularis within its tube. 

 o, operculum ; g, gills ; t, tube. 



